1. Technical Field
The present invention relates, generally, to methods and apparatus for applying a viscous substance to thin, flat sheets, and, more particularly, to an improved method of applying a uniform coating of electrochemically active paste to electric storage battery grids.
2. Background Art and Technical Problems
Lead storage batteries typically comprise a series of thin, flat, generally rectangular grids for carrying current through the battery. The grids also serve as a substrate for supporting the electrochemically active material, or paste, deposited thereon during manufacture. The paste typically comprises a mixture of lead oxide and a dilute acid solution, for example sulfuric acid. Paste composition is determined by, inter alia, power requirements, cost, and battery environment, as is known in the art.
Grids are desirably screenlike, having a matrix or honeycomb pattern of alternating metal strips and open spaces. The paste is deposited upon both surfaces of the grid, completely filling the spaces between the alternating strips ("interstitial spaces"). The pasted grids are then dried and assembled into a battery housing, whereupon the housing is sealed and filled with aqueous electrolyte.
Current flows through the pasted grids during the charge and discharge phases of battery operation. It has been observed that nonuniform current density within a battery creates localized volume expansion within the grids, causing the active material (paste) to buckle or crumble. Maintenance of uniform current distribution through the mass of active material (paste) requires that each grid support an equal amount of paste. It is, therefore, important that the paste be uniformly distributed onto the surfaces and within the interstitial spaces of the grids. Thus, an efficient method of producing pasted grids with consistent thickness dimensions is needed.
Grid pasting machines for applying a uniform coating of electrochemically active paste to lead battery grids are generally well known. For example, in the JCI Paster, Model #24727, made by Butler Tool Co. of Butler, Wis. 53007, grids are introduced into a pasting zone via a conveyor system of horizontally mounted cylindrical rollers. The pasting zone comprises the region between a pair of parallel flat plates. As each grid passes between the plates, paste is continuously fed into the pasting zone through a transverse slot in the upper plate. The paste enters the pasting zone under sufficient pressure to fill all interstitial spaces within the grid and completely coat both grid surfaces with paste.
The pasted grid is thereafter carried through a fixed orifice, comprising the sizing portion of the pasting zone, wherein the plates are spaced apart by an amount equal to the desired thickness of a pasted grid, and subsequently discharged from the paster machine.
Pressurized application of paste to the upper surface of the grids forces paste into and through the interstitial spaces of the grid. Consequently, the grids do not contact the orifice plates as the grids move through the sizing portion thereof. Rather, the pasted grids glide through the sizing orifice, there being a cushion of paste between the grid and each plate surface.
The rate at which paste is introduced into the pasting zone is selected such that a greater volume of paste is applied to each grid than is ultimately required on a completed pasted grid. In this way, excess paste is necessarily removed from the grid in the sizing orifice, thus minimizing the potential for undersized pasted grids.
Grids are typically introduced into the pasting zone either in the form of a continuous web, for subsequent cutting into individual segments, or in discrete units of one, two or four grids. In either case, each grid is pushed into the pasting zone in abutting relation to the upstream grids. An initial pair of drive wheels, similar to the ones protruding from the orifice plates, may be disposed near the entrance of the orifice plates for pushing the grids into the pasting zone. Consequently, the ease with which a grid may be pushed through the orifice plates, and particularly through the pinch point of the paste application zone, is a function of the beam strength of the grid. In this context, beam strength refers to the ability of a thin, flat grid to withstand compressive forces, particularly those applied in the plane of the grid as it is pushed through the pasting machine. Insufficient beam strengh results in grid buckling, which requires shutting down the machine to clear the resulting obstruction.
Depending upon, for example, voltage and current requirements of a particular battery, the thickness of the unpasted grids may vary. Relatively thick grids exhibit high beam strength. As the thickness of a grid decreases, a corresponding reduction in beam strength results.
Other factors contribute to lower grid beam strength such as, for example, the use of softer alloys, screen pattern design, and the temperature of the grid. In particular, when a web of grid material is produced by a continuous casting process, productivity may often be enhanced by disposing the grid casting apparatus adjacent the infeed conveyor of the pasting machine so that the grid web flows directly from the casting machine into the pasting machine. Freshly cast grids, having an elevated temperature, generally exhibit decreased beam strength.
It is generally known to carry, rather than push, a grid through the pasting zone to avoid buckling. See, for example, Yanik U.S. Pat. No. 4,606,383, issued Aug. 19, 1986, the disclosure of which is hereby incorporated by reference. This is accomplished by using a conveyor belt to "pull" the grids into and through the pasting zone. High pressure application of the paste to the grid, however, tends to cause the moist paste to adhere to the surface of the conveyor belt. One technique for preventing adherence involves inserting a web of paper between the grids and the belt. In this way, the pasted grid will adhere to the paper, thus facilitating a smooth transfer of the pasted grid and paper from the belt. Although the presence of the paper on one or both surfaces of a pasted grid does not adversely affect the electrochemical performance of the completed battery, paper presents awkward and expensive production and handling problems.
Another disadvantage of presently known belt drive pasting machines is that the belt functions as the bottom orifice plate. Inasmuch as belt thickness can vary considerably along the length of the belt, the orifice thickness dimension varies accordingly. Furthermore, this variation is exacerbated over time as the belt is worn and stretched during use. A variable orifice dimension results in poor repeatability of pasted grid thickness dimensions.